I would just add that it’s a gradient of transcription factor(s), proteins that regulate production of other proteins. Some of the regulated proteins are other thranscription factors that create finer gradients, like repeated stripes of gradients. along the initial, single gradient. This can be repeated on multiple levels so that pretty much every point of the body has unique combination of transcription factors. This combination triggers all the other proteins, including structural that create the form of the body. and of course, all this doesn’t just happen in 3D, but also changes with time. It’s complicated.
For illustration and possible direction for some particular interesting cases wrapped in a very entertaining package, I recommend this: www.youtube.com/watch?v=ydqReeTV_vk